Section
7 Wire ropes
7.1 General
7.1.1 This Section applies to steel wire ropes for positional mooring systems
of offshore units.
7.1.2 Wire ropes, associated fittings, are to be of an approved design.
7.2 Rope construction
7.2.1 When selecting a rope construction the following considerations apply:
- Required service life.
- Position in catenary.
- Axial stiffness properties of rope.
- Bending over sheaves, etc.
- Characteristic torsional properties of rope construction.
7.2.2 Various rope constructions can be accepted for long-term mooring
applications. These include:
- spiral strand.
- locked coil.
- six-strand.
Other constructions can be considered.
7.3 Design verification
7.3.1 The design of wire rope and associated fittings is to be verified. The
following information will be required for appraisal and information:
- Plans of rope assembly, components and fittings such as
terminations/sockets, bearings, pins and locking mechanism, bend stiffeners,
electrical insulation and other fittings.
- Materials specification covering all components and fittings,
steel wires, steel fittings, pins, socketing resins, sheathing and blocking
or lubricating compound).
- Corrosion control specification (anodes, steel wire
galvanisation, coating, electrical insulation, arrangement, and supporting
calculations.
- Design specification.
- Purchaser’s specification.
- Codes and Standards applied.
- Calculations for the strength and fatigue of rope, socket,
fittings, and their corrosion protection.
- Dimensions of rope assembly and components and fittings as well
as associated tolerances.
- Weight properties and tolerances (to include weight per metre of
main rope section inclusive of sheathing).
- Axial and torsional stiffness data.
- Wire rope datasheet (inclusive of all rope main characteristics,
tolerances, handling and service limiting criteria).
7.3.2 Data from prototype rope tests is to be submitted (inclusive of material,
construction and test procedures and data)..
7.3.3 Fatigue life calculations for steel wire ropes can be carried out in
accordance with a recommended Code, e.g., API RP 2SK: Recommended Practice for
Design and Analysis of Stationkeeping Systems for Floating Structures. Rope
bending fatigue effects are to be included where relevant.
7.3.5 The rope termination including the socket, socketing arrangement and pin
is to be designed to withstand a load of not less than the minimum breaking strength
of the attached wire rope and be shown to withstand no significant plastic
deformation under loading equal to 80 per cent of this load.
The rope is to be designed for the maximum design, storage and
installation conditions specified for the project.
7.3.6 Pin locking mechanism, anodes and bend stiffeners attachment to the
sockets should not be in the main load carrying path of the socket.
7.3.7 Sheathing should be able to match and accommodate the rope flexure and
elongation under rope design and service loads without loss of integrity. The
sheathing should not rotate in relation to the sockets and be designed for the
maximum grip pressure under various tensions specified for the project.
7.3.8 Bend stiffeners and their connection to sockets shall be designed to
protect the wire rope end termination from over bending under design loads
(including storage, installation and service) for the range of off line pull angles
or curvatures specified for the project.
7.3.9 Bend stiffener connections to sockets should be adequately protected
against corrosion.
7.4 Materials
7.4.1 Steel wire used for rope manufacture is to be manufactured in accordance
with a recognised National Standard:
- The steel is to be of homogeneous quality, consistent strength,
and free from visual defects likely to impair the performance of the
rope.
- The minimum tensile strength of the wire is to be the tensile
strength ordered. The maximum tensile strength is not to exceed the
specified minimum strength by more than 230 N/mm2. The tensile
strength should normally be within the range 1420 to 1770 N/mm2.
7.4.2 The material used in the manufacture of sockets is to comply with the
following requirements:
- Cast sockets:
- Castings are to be manufactured and tested generally in
accordance with Ch 4 Steel Castings of the Rules for the
Manufacture, Testing and Certification of Materials
(hereinafter referred to as the Rules for Materials).
- As a supplement to Ch 4 Steel Castings of the Rules for Materials,
impact tests are to be carried out at a test temperature of minus
20°C, to satisfy a minimum average energy requirement of 40J, with
no more than one individual result from each three test specimens
being less than 70 per cent of the required minimum average.
Increased material toughness may be required in specific cases.
- Alternative casting standards to Ch 4 Steel Castings of the Rules for Materials
complying with recognised national or proprietary specifications may
be accepted, see also
Ch 4, 1.1 Scope of the Rules for Materials.
- Fabricated sockets:
- Socket pins:
- Socket pins may be cast or forged. Where cast, material
requirements are to comply with (a) above. Forged socket pins are to
be manufactured in accordance with Ch 5 Steel Forgings of the Rules for Materials.
- As a supplement to Ch 5 Steel Forgings of the Rules for Materials,
impact tests are to be carried out at a test temperature of minus
20°C, to satisfy a minimum average energy requirement of 40J, with
no more than one individual result from each three test specimens
being less than 70 per cent of the required minimum average.
Increased material toughness may be required in specific cases.
- Alternative standards to Ch 5 Steel Forgings of the Rules for Materials,
complying with recognised national or proprietary specifications may
be accepted, see also
Ch 5, 1.1 Scope of the Rules for Materials.
7.5 Corrosion protection
7.5.1 Wire ropes are to be protected against corrosion. The corrosion
protection will normally consist of galvanising or other sacrificial coating of
individual wires. Filler wires of zinc or other suitable sacrificial material can be
incorporated in the outer layers of the rope, as an addition to, but not in place
of, galvanising of individual wires.
7.5.2 Galvanising is to meet the following minimum standards:
- Zinc:
- Zinc weight:
- ASTM A 603 Table 5, Class A (spiral strand and locked
coil).
- ISO 2232, Quality B (six-strand ropes).
- Alternative recognised Codes or Standards providing acceptable
equivalence will be considered.
7.5.3 Sockets are to be protected against corrosion by sacrificial anodes or
acceptable equivalent.
7.5.4 Suitable arrangements are to be made to insulate the corrosion protected
rope/socket assembly from adjacent non-protected elements in the system.
7.5.5 Polyethylene sheathing can also be used on appropriate rope
constructions, as an addition to, but not normally as an alternative to,
galvanising:
- Where sheathing is intended to be fitted, the specification is
to be submitted. ASTM D 1248 is an acceptable specification for medium or
high density polyethylene sheathing.
- A continuous strip of contrasting colour is to be incorporated
into the sheathing to aid monitoring for twist. The position of the strip
around the circumference sheathing in relation to a reference point on each
end socket should be reported on plan.
7.5.6 Compound used as blocking or lubricating material shall as a minimum
meet the requirements ISO 4346 Steel Wire Ropes for General Purposes - Lubricants -
Basic Requirements or equivalent.
7.5.7 Compound must not adversely affect the long term integrity of the wires
and sheathing.
7.6 Manufacture and testing
7.6.1 Steel wire ropes are to be manufactured in accordance with the design
standards and procedures and at a works approved by LR. Ropes and fittings will be
subject to LR survey during manufacture and testing.
A prototype testing programme is to be agreed with LR and carried out
under LR survey. Prototypes are to be of the same materials, construction and
termination unless specifically agreed with LR.
7.6.2 A certified ISO 9001/9002 Quality System is to be in place and a quality
plan is to be produced and agreed with LR’s Surveyors.
7.6.3 Where sheathing is specified, it is to be carried out in accordance with
the Quality Plan.
7.6.5 The following minimum requirements for the non-destructive testing of
cast sockets are applicable:
- Ultrasonics: All areas of all sockets and pins.
- Radiography: Critical areas of first, last, and one
intermediate socket selected by the LR Surveyor to be examined. Critical
areas to be identified on design drawings, and these to be included in the
design submission for verification.
- Magnetic Particle Inspection (MPI): 100 per cent of all
sockets and pins.
- Visual: 100 per cent of all sockets and pins.
7.6.7 Tests are to be carried out on individual wires for the following:
- Tensile strength and elongation.
- Torsion.
- Reverse bend.
- Zinc coating; mass, uniformity and adhesion.
Tests are to be carried out in accordance with recognised National
Standards such as ISO 2232, and ASTM A603, as appropriate.
7.6.8 Rope production samples are to be tested for the following:
- Modulus.
- Minimum breaking strength.
7.6.9 The tests required by Pt 3, Ch 10, 7.6 Manufacture and testing 7.6.8 are to be as follows:
- The modulus test is to be carried out on one finished rope
sample taken from the first production length. Production sockets need not
be fitted for this particular test. Load/extension characteristics and
permanent stretch are to be determined and documented. Acceptance criteria
for permanent stretch are to be as follows:
- Maximum of 0,4 per cent for spiral strand and locked
coil ropes.
- Maximum of 0,8 per cent for six-strand ropes.
The modulus of elasticity is to be calculated and
documented. The basis for the calculated value (cross-sectional metallic
area, or area of circle enclosing the rope) is to be clearly
stated.
- Breaking load test is to be carried out on one sample taken
from each manufactured length.
7.6.10 Socketing is to be performed according to the quality plan and is to
follow a tested and repeatable procedure drawing on ISO 17558 ‘Steel wire ropes -
Socketing procedures - Molten metal and resin socketing’ and be carried out by
experienced personnel.
Due attention should be paid to the following parameters:
- rope termination brush configuration,
- cleanliness of socket and rope brush,
- resin mix and mixing technique,
- brush and socket positioning and alignment,
- resin pouring technique,
- control of temperature and duration of curing,
- scale effect.
7.6.11 The characteristic mechanical properties (e.g. modulus of elasticity,
shrinkage, compressive strength) of the socketing resin shall be established from a
number of test samples. Guidance on tests methods can be drawn from BS 6319 Testing
of Resin Compositions for Use in Construction (especially Part 1, 2, 5 and 12).
- resin compressive strength and modulus of elasticity (also ISO
604 Plastics – Determination of compressive properties).
- shrinkage
- density and hardness (also EN 59 Glass reinforced plastics –
Measurement of hardness by means of a Barcol impressor).
The number of test samples is to be such as to establish the properties
with sufficient confidence
7.6.12 Bend stiffeners are to be manufactured according to a quality plan and
follow a qualified and repeatable procedure. Material properties (e.g. tear
strength, elasticity, water absorption, aging) are to be reported.
7.6.13 The maximum allowable curvature of the wire rope under storage, service
and installation conditions are to be substantiated and documented by the
manufacturers.
7.6.14 Material properties (e.g. elasticity and water absorption) of the
sheathing are to be documented. The sheathing should be manufactured such that it
does not rotate in relation to the wire rope and sockets. The manufacturer is to
substantiate and report curves of maximum allowable grip pressure under various
tensions for the sheathed rope provided.
7.6.15 Complete rope assembly characteristics and associated tolerances are to
be documented.
7.7 Identification
7.7.1 Each wire rope assembly is to be marked at each end with the letters LR
and the Certificate Number.
7.8 Certification
7.8.1 A certificate is to be issued for each rope assembly by LR. The
following is to be included in the Certificate:
- Purchaser’s name and order number.
- Description of order, including wire rope diameter and
construction.
- Tested minimum breaking load.
- Design Appraisal Document Number.
- Socket inspection certificate references.
- Individual wire certificate references.
- Sheathing report references.
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